The present invention is related to the field of portable light sources, such as flashlights.
The light output from a typical incandescent lamp of the type used in portable light sources is very sensitive to the voltage applied to the lamp. It has been determined that light output intensity changes in proportion to the voltage raised to the power of 3.5. Additionally, the voltage supplied by a typical dry cell battery, such as an alkaline battery, diminishes steadily throughout its operating lifetime. Consequently, the light output from a flashlight tends to become quite dim relatively early in the operating lifetime of the batteries. At this point the user must choose whether to continue using the same batteries, suffering the reduced light output for a significant period of operating time, or to replace the batteries to obtain better light output. The latter option increases the cost of operating the flashlight.
There are several known techniques for addressing the general problem of improving flashlight performance and/or efficiency. U.S. Pat. Nos. 5,498,934 and 5,418,433 to Nilssen, for example, show flashlights in which a BOOST switch and associated circuitry are used to temporarily increase light output by up to 300%. This feature can be used to obtain a markedly increased light output for very brief periods, as may be required by a user. However, the technique does not address the problem of maintaining consistent high-quality light output as batteries age.
U.S. Pat. No. 4,949,014 to Gasvoda, U.S. Pat. No. 4,499,525 to Mallory and U.S. Pat. No. 4,326,161 to Kreinberg show-battery-powered lamps in which the lamp intensity is regulated to be substantially constant despite the diminishing of battery voltage over an operating lifetime. In all of these lamps, the nominal battery voltage is substantially greater than the voltage required to obtain the desired light intensity. The lamp intensity is regulated by varying the duty cycle of an electronic switch inserted in series between the batteries and the lamp. The slow response time of the lamp element is exploited to integrate the energy pulses. These devices require relatively high-voltage batteries, and the lifetime of the lamps may be reduced due to the direct application of high-amplitude rectangular ripple currents to the lamps.